This entry is now old law. The new law is at http://www.immigration.com/blogs/i-140-ead-regulations-effective-17-jan…
5 November 2015: After some flip flops, USCIS has gone back to allowing carry forward of priority dates even if I-140 is revoked by employer - as described in this discussion. So, we are good for now.
USCIS recently began transferring some Form I-130, Petition for Alien Relative, cases filed by lawful permanent residents for their eligible family members from the Vermont Service Center to the California Service Center.
If your case was transferred, USCIS will send you a notice listing the transfer date and where your case will be processed. Your original receipt number will not change and this will not delay the processing of your cases except for the additional time needed to transfer the file.